1. Field of the Invention
The invention generally relates to performance features of a wing of an aircraft. More particularly, the invention relates to a structure for inhibiting cross flow instabilities from reducing the performance of a wing or other aerodynamic surfaces of an aircraft.
2. Description of the Related Art
Over the years, wing design has changed dramatically. Wing leading edges are swept to allow higher speeds without suffering large drag increases. Wings and tails are also swept to control the aerodynamic center and balance of an aircraft. Other issues including integration of sensors also drive the wing designer to sweep the wing leading edge back.
An important issue to the design of the swept wing is cross flow instabilities. Cross flow is the flow of air along the wingspan, from the root towards the tip, as opposed to over or under the wing, as it is designed to travel across a wing. Cross flow is not parallel to the primary air flow direction (the direction of travel of an aircraft), but flows outwardly, towards the wing tips when the wing is swept back. Cross flow occurs very close to the wing surface in an area referred to as the boundary layer. The air in the boundary layer is heavily influenced by the effects of viscosity and the ‘no-slip’ condition at the surface of the wing. These effects retard the flow of air over the wing and create a “viscous” drag on the wing. The airflow outside the boundary layer (further from the wing surface) is only minimally influenced by the effects of viscosity.
On swept wings, cross flow occurs primarily in the boundary layer, but does not occur to the same magnitude in the inviscid region outside the boundary layer. There is a continuous rapid change in the direction of the flow inside the boundary layer with the maximum cross flow occurring just off the surface and reduced cross flow as the distance from the surface is increased. This change in the direction of the airflow with distance normal to the surface creates vorticity that is amplified downstream and causes the flow in the boundary layer to transition from laminar to turbulent. This transition is marked by a change in the flow character and the drag. The laminar boundary layer is ordered, and minimal mixing occurs between layers (lamina). A turbulent boundary layer is marked by turbulent mixing that disrupts the previous laminar flow. The turbulent mixing causes an increased rate of exchange of momentum between the higher velocity flow further from the surface and the lower velocity flow closer to the surface. This increased exchange of momentum creates larger velocities closer to the surface and this leads to higher ‘friction drag’ at the surface. The friction drag of a laminar boundary layer can be about half of the friction drag of a turbulent boundary layer and for a typical all wing subsonic aircraft, this results in about 25% lower total drag and 25% lower fuel consumption. The benefits are smaller if laminar flow is achieved only on the wings and the aircraft consist of a wing and fuselage.
To minimize the occurrences in which cross flow instabilities are amplified and cause transition from laminar to turbulent flow, Distributed Roughness Elements (DRE) have been designed into wings. DREs are physical “bumps” added to or designed into a surface of a wing. The physical bumps create a disturbance in the flow field that prevents cross flow instabilities from growing and causing the transition from laminar flow to turbulent flow boundary layer conditions. The bumps create periodic vorticity at a scale that is well damped downstream. This vorticity inhibits the formation of larger scale vorticity that is not damped and would grow and eventually cause the flow to transition from laminar to turbulent. In the past, these DREs have either been fixed geometric bumps in the wing surface or pneumatically powered flexible bumps. Both of these two solutions have their deficiencies. With regard to the fixed physical DREs, there is no control in the magnitude, spacing or disturbance location as these are fixed in place and made during the manufacturer of the wing or applied as an appliqué before flight. The pneumatically controlled DREs are complex, require a fluid source (air or the like), and offer limited control. The shape of pneumatic bumps is typically far from ideal also as the bumps tend to be smooth while sharper disturbances create more voracity.